The present invention relates to a receiver for receiving a television signal radiated from a broadcasting satellite, and more particularly to an audio processor in such a receiver for processing a plurality of frequency-modulated subcarriers contained in the television signal.
In recent years, broadcasting programs are supplied to a CATV via a broadcasting satellite. As shown in FIG. 3 of the accompanying drawings, a receiver used in such a CATV system has a parabolic antenna 1 which receives a bandwidth signal containing channel signals in a band of 3.7 to 4.2 GHz transmitted from the broadcasting satellite. A low-noise block converter (LNB) 2 coupled to the antenna 1 converts, by way of group conversion, the received signal into a bandwidth signal in the range of from 950 to 1450 MHz. Thus, the received signal is converted into a signal of a first intermediate frequency (first IF signal).
The first IF signal is then transmitted over a coaxial cable 3 to a channel selector 4 in an indoor section of the receiver. The channel selector 4 selects a desired television signal out of a plurality of channels having an allowable occupied bandwidth such as 36 MHz. The selected television signal is converted by the channel selector 4 into a signal of a second intermediate frequency (600 MHz, for example), from which noise is removed by a bandwidth filter 5. The signal is then demodulated by a frequency demodulator 6 to produce a baseband signal. The baseband signal is applied to both a video signal processor 7 and an audio signal processor 8. The video signal processor 7 produces a video signal, while the audio signal processor 8 produces an audio signal.
As shown in FIG. 4, the baseband signal contains a plurality of audio subcarriers b (up to 16 subcarriers) in a bandwidth of 5 MHz to 8.5 MHz for one channel. The band is utilized in different conversion formats by different transmitting stations. More specifically, there are no standards established for the format of the multiplexing conversion system and the maximum frequency deviation of the audio subcarriers. Therefore, it has been customary for the operator for the CATV system to switch formats and set the frequency deviation to a prescribed value while referring to a list of programs previously distributed for each channel. Designated in FIG. 4 at a is a video signal with its low-frequency-range level lowered due to preemphasis effected in the transmitting station.
For processing a single (fixed) subcarrier in the audio signal processor 8, it is necessary to use only a single bandpass filter 9 corresponding to the audio subcarrier to be processed, as illustrated in FIG. 5. Where the audio subcarrier has a central frequency of 6.8 MHz with a maximum frequency deviation of .+-.75 KHz, a bandpass filter having a central frequency of 6.8 MHz and a bandwidth corresponding to the frequency deviation should be used. Denoted in FIG. 5 at 10 is an audio demodulator, 11 a deemphasis circuit, 12 a volume control, and 13 an audio amplifier.
Where a plurality of audio subcarriers with different maximum frequency deviations are to be selected, it would be possible as shown in FIG. 6 to employ a frequency converter composed of a frequency mixer 14 and a voltage-controlled oscillator 15 for converting each of the audio subcarriers into an IF signal having a central frequency of 10.7 MHz, and to pass the IF signal through a bandpass ceramic filter 16 having a central frequency of 10.7 MHz and a bandwidth of .+-.75 KHz, with the resulting signal being demodulated by an audio demodulator 10.
Even if the audio subcarriers with different maximum frequency deviations were converted into the IF signal, however, the following problem would occur by passing the IF signal through the sole bandpass ceramic filter 16: For converting an audio subcarrier having a large maximum frequency deviation into an IF signal, the maximum frequency deviation of the IF signal would be greater than the bandwidth of the filter 16, and hence the S/N ratio of a demodulated audio signal would be increased. For converting an audio subcarrier having a small maximum frequency deviation into an IF signal, the maximum frequency deviation of the IF signal would be smaller than the bandwidth of the filter 16, resulting in a THD (Total Harmonic Distortion) to which a demodulated audio signal is subjected.
One conventional solution has been to provide, as shown in FIG. 7, a wide-bandpass filter 17 and a narrow-bandpass filter 18 parallel to the bandpass filter 16, and to select these filters 16, 17 and 18 with a switch 19 dependent on audio subcarriers with different maximum frequency deviations for thereby producing an audio signal for good sound quality.
However, where IF signals with different maximum frequency deviations are demodulated by a plurality of filters having different bandwidths, the levels of demodulated signals vary with the frequency deviations, and hence the level of a reproduced sound has to be adjusted each time one of the audio subcarriers is selected. Therefore, controlling the receiver has been tedious and time-consuming. With the filters of different bandwidths used for the respective audio subcarriers, the receiver is complicated in construction and costly to manufacture.